An aerodynamic bearing, or air bearing, is a bearing that supports the load it is subjected to via a pressurized mass of air that is introduced between a rotor rotating at high speed and a stator. Bearings of this type are used, for example, in applications in which the rotors rotate at high speeds, where loads are generally limited and/or high precision is required. The use of air instead of a fluid such as oil to create lubrication between the rotor and stator serves to limit the power dissipated by fluid friction, because the viscosity of air is much less than that of an oil, even if the oil is not very viscous.
To provide lubrication during the startup and shutdown phases, when the air pressure between the rotor and the stator is insufficient for maintaining a bearing film between these two parts, it is known to introduce a foil structure between the rotor and the stator to provide lubrication at low rotation speeds with minimal disruption to the operation of the bearing at high speeds.
US document 2005/0201646 discloses an air bearing with a foil sheet forming a closed loop with several tops and a corresponding number of bulged arcuate surfaces. An elastic or viscoelastic material is filled in the air gap between the foil and a corresponding stator. In that document, the geometry of the foil bearing forms a naturally converging shape without requiring deformation of the flexible structure of the bearing. The elastic or viscoelastic material provides an elastic quality and/or a damping quality.
Document WO-2011/025087 shows a foil-air bearing having a top foil along the inner periphery of the hole of the bearing housing. A corrugated foil mounted on stiffeners (called a “bump foil”) is placed between the top foil and the inner surface of the hole. The bump foil provides both stiffness and damping in the association between the rotor and its bearing.
Document KR-20040029700 implements a combination of a structure similar to that illustrated in WO-2011/025087 and a viscoelastic foil which comes above the top foil and is therefore intended to come in contact with the rotor. The structure proposed here allows a higher rotational speed while damping the vibrations of the rotor.
The present invention aims to provide a bearing that achieves both a viscous damping and a frictional damping but the two damping effects are decoupled. Advantageously, a bearing according to the invention will at least partly absorb the subsynchronous vibratory phenomena that can be observed in the aerodynamic bearings of the prior art.
Preferably, the invention proposes a structure that is simple to implement and to apply.